1. Field of the Invention
The invention relates to an optical beam transformation system for transforming an entrance light distribution striking an entrance surface of the optical beam transformation system into an exit light distribution emerging from an exit surface of the optical beam transformation system by radial redistribution of light intensity, as well as to an illuminating system for an optical device which includes at least one such optical beam transformation system. The optical device including the illuminating system may be a projection exposure apparatus for microlithography.
2. Description of the Related Art
The efficiency of projection exposure apparatuses for the microlithographic production of semiconductor components and other finely structured subassemblies is determined substantially by the imaging properties of the optical projection system. Moreover, the image quality and the wafer throughput achievable with an apparatus are substantially codetermined by properties of the illuminating system connected upstream of the projection objective. The said system must be capable of preparing the light of a light source with the highest possible efficiency and in the process setting a light distribution which can be precisely defined with reference to the position and shape of illuminated regions, and in the case of which as uniform as possible a distribution of intensity is present within illuminated regions. These requirements are to be fulfilled in equal measure for all settable illuminating modes, for example for conventional settings with various degrees of coherence or in the case of an annular field, dipole or quadrupole illumination.
A requirement becoming ever more important which is placed on illuminating systems consists in that the latter are to be capable of providing output light with a state of polarization which can be defined as precisely as possible. For example, it may be desired for the light falling onto the photomask or into the downstream projection objective to be largely or completely linearly polarized and to have a defined alignment of the preferred direction of polarization. Modern catadioptric projection objectives with a polarization beam splitter (beam splitter cube, BSC) having a theoretical efficiency of 100% at the beam splitter, for example, can operate with linearly polarized input light.
If the illuminating system is used in conjunction with an excimer laser light source, which already provides largely linearly polarized light, linearly polarized output light can be provided by virtue of the fact that the overall illuminating system operates substantially in a fashion maintaining polarization. An optical system operates “in a fashion maintaining polarization” in the meaning of this application when the state of polarization of the light emerging from the optical system corresponds substantially to the state of polarization of the light entering the optical system.
Illuminating systems, in particular those used for microlithography projection exposure apparatuses, normally have a complex design with a multiplicity of different subsystems and components for various functionalities. If it is desired in the case of one illuminating system to be able to switch over between conventional (axial, on-axis) illumination and nonconventional (abaxial, off-axis) illumination, use is preferably made for this purpose of optical beam transformation system, such as axicon systems, which are capable of transforming by radial redistribution of light intensity an entrance light distribution striking an entrance surface of the optical beam transformation system into an exit light distribution in the case of which the light intensity outside the optical axis is substantially greater than in the region of the optical axis. These nonconventional illumination settings for producing an abaxial, oblique illumination can serve the purpose, inter alia, of increasing the depth of field by means of two-beam interference, and of increasing the resolving power of projection exposure apparatuses.
EP 747 772 describes an illuminating system having a combined zoom-axicon objective in the object plane of which a first diffractive raster element with a two-dimensional raster structure is arranged. This raster element serves the purpose of increasing the geometrical flux of the striking laser radiation by introducing aperture, and of varying the form of the light distribution such that, for example, an approximated circular distribution (for conventional illumination) or a polar distribution results. In order to alternate between these illumination modes, first raster elements are exchanged as appropriate. The zoom-axicon objective combines a zoom function for infinite adjustment of the diameter of a light distribution with an axicon function for radial redistribution of light intensities. The axicon system has two axicon elements which can be displaced axially relative to one another and have mutually facing conical axicon surfaces which can be moved towards one another until their spacing is zero. Consequently, the annularity of the illumination and the degree of coherence can be adjusted by adjusting the zoom axicon. A second raster element, which is located in the exit pupil of the objective, is illuminated with the corresponding (axial or abaxial) light distribution, and forms a rectangular light distribution whose shape corresponds to the entrance surface of a downstream rod integrator.
Other illuminating systems with axicon systems for radial redistribution of optical energy are shown, for example, in U.S. Pat. No. 5,675,401 belonging to the applicant, in U.S. Pat. No. 6,377,336 B1, nd in parallel property rights or in U.S. Pat. No. 6,452,663 B1.
The invention also relates to an illuminating system for a microlithography projection exposure apparatus, comprising an axicon module. The axicon module acts as an optical beam transformation system.
Such illuminating systems are disclosed, for example, in DE 44 21 053 (U.S. Pat. No. 5,675,401) or DE 195 20 563 (U.S. Pat. No. 6,258,443). In particular, the references cited in DE 44 21 053 (U.S. Pat. No. 5,675,401) specify known axicon modules in illuminating systems.
An axicon module may have first axicon element with a first axicon surface, and a second axicon element, assigned to the first axicon element, with a second axicon surface. When the two axicon surfaces are arranged along an optical axis at a spacing, the axicon module produces an illumination distribution with a central intensity minimum. The axicon module is arranged as a rule in the illuminating system such that the exit pupil of the illuminating system has the illumination with the central intensity minimum. An annular illumination results in the case of conical axicon surfaces. The diameter of the annular illumination can be changed by varying the spacing of the two axicon elements. A multipole illumination is produced if the axicon surfaces are respectively formed from individual segments that are arranged pyramidally, that is to say form the roof of a multihedral pyramid, as it were. The quadrupole illumination frequently used in lithography results in the case of four segments. In the case of multipole illumination, as well, it is possible to vary the spacing of the illuminated regions from the optical axis by varying the spacing of the two pyramidal axicon elements. The lithographic transfer of a mask pattern onto the substrate to be exposed can be optimized by varying the illumination distribution in the exit pupil of an illuminating system for microlithography projection exposure apparatuses. The two axicon surfaces of the mutually assigned axicon elements are generally concave-convex or convex-concave.
Pairs of axicon elements arranged one behind another and with conical and pyramidal axicon surfaces are also known from EP 0 949 541. The spacing of the axicon elements is variable in each case here.
DE 195 35 392 (U.S. Pat. No. 6,191,880) discloses an illuminating system for a microlithography projection exposure apparatus having an axicon module. Furthermore, the illuminating system has a polarization-influencing optical element in order to polarize rays radially in relation to the optical axis of the illuminating system. However, in the exemplary embodiment illustrated in FIG. 5, the polarization-influencing optical element is arranged at the earliest downstream of the axicon module. Consequently, the rays with the polarization state prescribed by the light source strike the axicon surfaces. The lasers used in microlithography at DUV wavelengths generally produce linearly polarized light. In accordance with DE 195 35 392 the polarization-influencing optical element is preferably arranged at the earliest downstream of the last asymmetric element such as, for example, deflecting mirror or polarization beam splitter layers. Otherwise, the radial polarization that is desired for the optimum incoupling of the rays into the wafer resist is lost again.
A similar illuminating system to that from DE 195 35 392 (U.S. Pat. No. 6,191,880) is disclosed in DE 100 10 131 (US 2001/0019404). In this case, too, the polarization-influencing optical element is arranged at the earliest downstream of the axicon module. Consequently, in the illuminating system of DE 100 10 131 as well the rays with the polarization state prescribed by the light source strike the axicon surfaces. The polarization-influencing optical element produces tangential polarization in DE 100 10 131. The tangential polarization of the rays improves the two-beam interference during the production of images. In accordance with the disclosure in DE 100 10 131, it is necessary for the polarization-influencing optical element to be arranged at the earliest downstream of the last asymmetric element such as, for example, deflecting mirror or polarization beam splitter layers. Otherwise, the tangential polarization is lost again.
The light sources used in microlithography projection exposure apparatuses generally produce linearly polarized or unpolarized light. The latter then strikes the axicon surfaces of the axicon module. The axicon surfaces have optical surfaces that are inclined with reference to the optical axis. This results at the axicon surfaces in reflection losses that depend on the polarization state of the rays, as will be explained below. The polarization component whose electric vector vibrates parallel to the incidence plane of a ray is denoted below as p-component. Correspondingly, the polarization component with electric E-field vector vibrating perpendicular to the incidence plane of a ray is denoted below as s-component. Consideration is given to an axicon module that is arranged along an optical axis running in the z-direction. The axicon surfaces each comprise four segments of a pyramidal structure that are inclined at the Brewster angle. The pyramidal structure is aligned in the x/y-direction. The axicon surfaces are intended to have no antireflection coating. The incident rays may be linearly polarized in the y-direction. The rays are now reflected at the axicon surfaces as a function of polarization in accordance with the Fresnel formulas, or refracted. The p-polarized rays are refracted at the segments, arranged in the positive and negative y-directions, of the axicon surfaces without reflection losses, while the s-polarized rays suffer reflection losses at the segments, arranged in the positive and negative x-directions, of the axicon surfaces. As a result of this, downstream of the axicon module the rays impinging along the y-axis have a higher intensity than the rays impinging along the x-axis. The intensity distribution is therefore nonuniform downstream of the axicon module and greater in the quadrants arranged along the y-axes than in the quadrants arranged along the x-axis. Consequently, the individual illuminated regions have total intensities of different magnitude. However, because of the polarization-dependent refraction/reflection a nonuniform intensity distribution results even with a suitable antireflection coating or with an adaptation of the inclination angle of the axicon surfaces. Consequently, structures are imaged differently as a function of their orientation during the lithography process. In addition, with conical axicon surfaces there is also a change in the polarization state for rays that do not strike the axicon surfaces along the x-axis and the y-axis. The change in the linear polarization state of a ray can lead to further losses at downstream deflecting mirrors or polarization-dependent beam splitter layers.
Conventional beam transformation systems, such as axicon systems, generally do not operate to maintain polarization. Because of the rotationally symmetrical or radially symmetrical geometry of the axicon surfaces with refracting surfaces inclined obliquely to the optical axis, in the case of linearly polarized input radiation, for example, beams with an identical direction of oscillation of the electric field vector are not incident everywhere in identically oriented incident planes with reference to the refracting axicon surfaces. Consequently, because of Fresnel losses, the entering light experiences an attenuation, dependent on the location of incidence and therefore differing locally, of the p- or s-components of the electric field strength. Here, the s-component is that electric field strength component which runs perpendicular to the plane of incidence which is defined by the surface normal to the axicon surface at the striking location and by the beam entrance direction. The p-component is the electric field strength component which oscillates parallel to the plane of incidence, that is to say in the plane of incidence itself. Consequently, incidences of attenuation can occur which vary on axicon surfaces in the azimuthal direction (circumferential direction). This leads in the case of homogeneous polarized entrance light to a polarization of the light distribution downstream of the axicon system which is no longer homogeneous. In the case of conventional axicon systems and linearly polarized input light, the loss in the linear degree of polarization can certainly be of the order of magnitude of up to approximately 10%.
It is known that axicon systems can have a polarizing effect. DE 35 23 641 (corresponding to U.S. Pat. No. 4,755,027) describes a polarizer that uses the polarization-selective effect of a number of consecutive axicon surfaces in order to produce tangential or radial polarization. The polarization-selective effect produced by the axicon surfaces inclined obliquely to the optical axis is amplified in the case of some embodiments by suitable optical coatings. Another polarizer with conical surfaces is shown in DE 195 35 392 A1 (corresponding to U.S. Pat. No. 6,191,880 B1).